A material is said to be frangible if it tends to break up into fragments rather than deforming plastically and retaining its cohesion as a single object. Frangible bullets are designed to intentionally disintegrate into particles upon impact with a surface harder than the bullet itself. Uses include firing range safety, to limit environmental impact, or to limit the danger behind an intended target. For example, frangible bullets are often used by shooters engaging in close-quarter practice or combat training to avoid ricochets. Frangible bullets are typically made of non-toxic metals, and are frequently used on “green” ranges and outdoor ranges where lead abatement is a concern.
An early example of a frangible bullet is the Glaser safety slug, which was originally a hand-made hollow point bullet filled with birdshot and covered with a flat polymer cap. To improve ballistic performance, a polymer-tipped round ball was introduced in 1987, and the current compressed core form was first sold in 1988. The formulation of the polymer was also changed in 1994 to improve fragmentation reliability. Compared to conventional ammunition, the rounds are said to be very expensive and less accurate.
Over the years, numerous alternative frangible bullet designs have emerged, some of which have become commercially available. SinterFire Inc. of Kersey, Pa., for example, owner of U.S. Pat. No. 6,263,798, manufactures and sells frangible bullets based upon a mixture of copper, tin and a metal or metalloid binder material which is compacted into a desired shape then heated and cooled.
Another example is AccuTec USA of Virginia Beach, Va., which markets and sells a frangible projectile purportedly having a specific gravity similar to that of lead. According to its U.S. Pat. No. 7,353,756, projectile comprises, by weight, 6-66% ballast and 34-94% polyether block amide resin binder. The ballast comprises at least one member selected from a group consisting of tungsten, tungsten carbide, molybdenum, tantalum, ferro-tungsten, copper, bismuth, iron, steel, brass, aluminum bronze, beryllium copper, tin, aluminum, titanium, zinc, nickel silver alloy, cupronickel and nickel.
While some frangible bullet designs utilize non-metallic or polymeric binders, others use ceramic materials. As one example, U.S. Pat. No. 5,078,054 teaches a frangible projectile made from powdered metals comprising a body of either iron and carbon, or of iron and alumina. The powdered metals are compacted, sintered, and cooled. A further example is disclosed by Abrams et al., U.S. Pat. No. 6,074,454, assigned to Delta Frangible Ammunition, LLC of Stafford, Va. The bullets in this case are typically made from copper or copper alloy powders (including brass, bronze and dispersion strengthened copper) which are pressed and then sintered under conditions so as to obtain bullets with the desired level of frangibility. The bullets also contain several additives that increase or decrease their frangibility. Such additives may include oxides, solid lubricants such as graphite, nitrides such as BN, SiN, AlN, etc., carbides such as WC, SiC, TiC, NbC, etc., and borides such as TiB2, ZrB2, CaB6.